Electronic compasses have been widely used in various electronic devices to improve performance thereof. For example, the electronic compass can be used in a global positioning system (GPS) to improve sensing capability. An advancing direction in the GPS is determined through movement of an object. However, when the object has a low speed, or is in a static state, the GPS cannot accurately orientation the object. The electronic compass can provide azimuth information to help determining the direction.
Various mechanisms of sensing a magnetic field have been provided, for example, a typical Hall device or a magneto-resistance device. The magneto-resistance device includes an anisotropic magneto-resistor (AMR), a giant magneto-resistor (GMR) and a tunneling magneto-resistor (TMR), which have higher sensitivity than that of the Hall device, and the back-end process is easy to be integrated with the front-end process of a complementary metal oxide semiconductor (CMOS).
The AMR magnetic field sensor has been commercialized, though it is limited to a 2-axis integrated chip type. The AMR may use 45 degree of shorting bars, so-called Barber pole bias structure, and work in a bipolar mode. The GMR has a larger magneto-resistance ratio (MR) than that of the AMR, though the GMR is hard to operate under the bipolar mode, and generally uses a unipolar mode to sense magnitude of the magnetic field. In recent years, implementation of the TMR with high magneto-resistance ratio draws greater attention, and only few single-axis magnetic field sensor products are produced for sale. Unexpectedly, the characteristics of the TMR and magnetic thin film limit feasibility of multi-axis magnetic field sensors thereof.
FIG. 1A and FIG. 1B are schematic diagrams of a typical TMR applied to a magnetic field sensor 95, which includes a bottom electrode 102 formed on a substrate 90, wherein a bottom plate formed by conductive metal serves as the bottom electrode 102; a magnetic tunneling junction (MTJ) device 110 formed on the bottom electrode 102; and a top electrode 106 formed on the MTJ device 110, wherein a top plate formed by conductive material serves as the top electrode 106. Cross lines intersected at a center can be defined from a structure pattern of the MTJ device, wherein a longer line thereof is referred to as a major axis 101, and a shorter line is referred to as a minor axis 103. Moreover, a line referred to as an easy-axis 180 is collinear with the major axis 101. The MTJ device 110 includes a pinned layer 112, a tunneling layer 115 and a free layer 116, wherein the MTJ device 110 is disposed between the bottom electrode 102 and the top electrode 106. The pinned layer 112 of a magnetic material is formed on the bottom electrode 102, and has a pinned magnetization 114 parallel to a pinned direction. The tunneling layer 115 of a non-magnetic material is formed on the pinned layer 112. The free layer 116 of the magnetic material is formed on the tunneling layer 115, and has a free magnetization 118, being initially parallel to the easy-axis 180.
After the MTJ device 110 is formed, for example, after magnetic thin film stacking and pattern etching, a magnetic field with a direction orthogonal to the easy-axis 180 is exerted during an annealing process. After the annealing process, the pinned magnetization 114 will be parallel to the direction of the applied magnetic field, and shape anisotropy of the MTJ device 110 makes the free magnetization 118 tend to be parallel to the easy-axis 180. Therefore, the magnetic field sensing direction of the TMR is orthogonal to the easy-axis 180 on the substrate after annealing process. Moreover, a magnetic film layer of a horizontally polarized material generally has an extremely strong demagnetization field, which confine the activities of magnetizations of the free layer and the pinned layers, all of which are in-plane of the magnetic film. Namely, the magnetizations are hard to stand on the horizontal plane but easy to rotate on the horizontal plane of the magnetic thin film. Therefore, the typical structure of the TMR is only adapted to an in-plane magnetic field sensor.
Due to the limitation of the magnetic thin film, if the magneto-resistor is used to sense the magnetic field with the direction orthogonal to the substrate, the magneto-resistor is generally fabricated on an incline of the substrate to sense a component of the magnetic field on the incline. A challenge of the AMR lies in that it requires a large incline area, and lithography and etching process of the screw pattern bar pole of 45 degrees is a great challenge. The direction of the pinned magnetization of the TMR is limited by the direction of the magnetic field of the annealing process, and integrated multi-axis magnetic field sensor cannot be fabricated.
The electronic compass is generally required to sense components of geomagnetic field on X-Y-Z directions. So far, a conventional electronic compass chip generally includes two or three 2-axis/single-axis magnetic field sensors to respectively sense the component of the geomagnetic field on each direction. Therefore, how to design a 3-axis integrated and low-cost magnetic field sensor is always a very popular topic in the field.